Method And Apparatus For The Monitoring And Control Of A Process

ABSTRACT

A catch side optical system for use in diode laser spectroscopy consisting of a catch side optic optically coupled to a catch side multimode optical fiber and means to mechanically manipulate a section of the catch side multimode optical fiber to minimize catch side mode noise. The mechanical manipulation may consist of twisting the catch side multimode optical fiber around its longitudinal axis. The means to mechanically manipulate the section of the catch side multimode optical fiber in the above fashion may consist of a motor associated with the catch side multimode optical fiber such that a section of fiber is held fast relative to a shaft position of the motor and the motor shaft is repetitively swept through +360 degrees and −360 degrees of motion. The frequency of the motor shaft sweep may be at least 10 Hz to enable effective averaging of the transmitted signal and thereby reduce the effect of catch side mode noise.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/543,288 filed on Jul. 25, 2005 entitled “Method And Apparatus For TheMonitoring And Control Of Combustion”, which is a 371 of internationalpatent application no. PCT/US04/10048 filed on Mar. 31, 2004 entitled“Method And Apparatus For The Monitoring And Control Of Combustion”,which is a nonprovisional of U.S. Provisional Patent Application No.60/459,108 filed on Mar. 31, 2003 entitled “Echelle Grating OpticalMultiplexer with Widely Spaced Wavelengths”.

TECHNICAL FIELD

The present invention is directed toward a method and apparatus for themonitoring and control of a process, and more particularly toward theuse of tunable diode laser absorption spectroscopy to monitor andcontrol combustion processes.

BACKGROUND ART

A large percentage of the electrical power generated in the UnitedStates of America is created in coal combustion power plants. The bulkof worldwide electricity production similarly relies on coal as aprimary energy source. It is likely that coal will remain a primaryenergy source in the foreseeable future given the long termenvironmental concerns with the storage of waste from nuclear energygeneration operations, and the inefficiencies associated with solarpowered electrical generation. In addition vast worldwide coal reservesexist sufficient for at least 200 years of energy production at currentrates.

However, there is and will continue to be a high demand to reduce theemissions of pollutants associated with coal fired electrical energygeneration, and to increase the overall efficiency of the coal firedgeneration process. Traditionally, in power plants and other industrialcombustion settings the efficiency of the combustion process and thelevel of pollution emission have been determined indirectly throughmeasurements taken on extracted gas samples with techniques such asnon-dispersive infrared (NDIR) photometry. Extractive sampling systemsare not particularly well suited to closed loop control of a combustionprocess since a significant delay can be introduced between the time ofgas extraction and the ultimate analysis. In addition, extractiveprocesses generally result in a single point measurement which may ormay not be representative of the actual concentration of the measuredspecies within what can be a highly variable and dynamic combustionprocess chamber.

Laser based optical species sensors have recently been implemented toaddress the concerns associated with extraction measurement techniques.Laser based measurement techniques can be implemented in situ and offerthe further advantage of high speed feedback suitable for dynamicprocess control. A particularly promising technique for measuringcombustion gas composition, temperature and other combustion parametersis tunable diode laser absorption spectroscopy (TDLAS). TDLAS istypically implemented with diode lasers operating in the near-infraredand mid-infrared spectral regions. Suitable lasers have been extensivelydeveloped for use in the telecommunications industry and are, therefore,readily available for TDLAS applications. Various techniques of TDLASwhich are more or less suitable for the sensing and control ofcombustion processes have been developed. Commonly known techniques arewavelength modulation spectroscopy, frequency modulation spectroscopyand direct absorption spectroscopy. Each of these techniques is basedupon a predetermined relationship between the quantity and nature oflaser light received by a detector after the light has been transmittedthrough a combustion process chamber and absorbed in specific spectralbands which are characteristic of the gases present in the process orcombustion chamber. The absorption spectrum received by the detector isused to determine the quantity of the gas species under analysis plusassociated combustion parameters such as temperature.

For example, Von Drasek, et al., U.S. Patent Application Ser. No.2002/0031737A1, teaches a method and apparatus of using tunable diodelasers for the monitoring and/or control of high temperature processes.Von Drasek features the use of direct absorption spectroscopy todetermine the relative concentration of numerous combustion species,temperature and other parameters. Calabro, U.S. Pat. No. 5,813,767,teaches a similar system for monitoring combustion and pollutantsdeveloped in a combustion chamber. Calabro utilizes an indirectspectroscopy technique wherein observed Doppler broadening of the shapeof an absorption feature serves as the basis for temperature analysis.

Teichert, Fernholz, and Ebert have extended the use of TDLAS as a knownlaboratory analysis technique to a workable field solution suitable forthe sensing of certain combustion parameters within the boiler fireballof a full sized coal fired power plant. In their article, “Simultaneousin situ Measurement of CO, H.sub.2O, and Gas Temperature in aFull-Sized, Coal-Fired Power Plant by Near-Infrared Diode Lasers,”(Applied Optics, 42(12):2043, 20 Apr. 2003) the authors present asuccessful implementation of direct absorption spectroscopy at a coalfired power plant and discuss certain technical challenges resultingfrom the extremely large scale and violent nature of the coal burningprocess. In particular, typical coal fired power plants have combustionchamber diameters of 10-20 meters. The plants are fired by pulverizedcoal, which results in a combustion process which both obscures thetransmission of laser light because of the high dust load and which isextremely luminous. In addition, various strong disturbances are foundunder power plant combustion conditions. The overall transmission rateof light through the process chamber will fluctuate dramatically overtime as a result of broadband absorption, scattering by particles orbeam steering owing to refractive-index fluctuations. There is alsointense thermal background radiation from the burning coal particleswhich can interfere with detector signals. The environment outside ofthe power plant boiler also makes the implementation of a TDLAS sensingor control system problematic. For example, any electronics, optics orother sensitive spectroscopy components must be positioned away fromintense heat, or adequately shielded and cooled. Even though theimplementation of a TDLAS system is extremely difficult under theseconditions, TDLAS is particularly well suited to monitor and control acoal combustion process. The present invention is directed to overcomingone or more of the TDLAS implementation problems discussed above.

SUMMARY OF THE INVENTION

One embodiment is a catch side optical system for use in diode laserspectroscopy consisting of a catch side optic optically coupled to acatch side multimode optical fiber and means to mechanically manipulatea section of the catch side multimode optical fiber to minimize catchside mode noise. The mechanical manipulation may consist of twisting thecatch side multimode optical fiber around its longitudinal axis. Themeans to mechanically manipulate the section of the catch side multimodeoptical fiber in the above fashion may consist of a motor associatedwith the catch side multimode optical fiber such that a section of fiberis held fast relative to a shaft position of the motor and the motorshaft is repetitively swept through +360 degrees and −360 degrees ofmotion. The frequency of the motor shaft sweep may be at least 10 Hz toenable effective averaging of the transmitted signal and thereby reducethe effect of catch side mode noise.

Another embodiment is a method of coupling light to a catch sidemultimode optical fiber used in a diode laser spectroscopy system. Themethod includes mechanically manipulating a section of the catch sidemultimode fiber to reduce catch side mode noise. The mechanicalmanipulation step may include but is not limited to twisting themultimode optical fiber around a longitudinal axis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a sensing apparatus of the presentinvention.

FIG. 2 is a schematic diagram of a sensing apparatus of the presentinvention featuring remotely located components optically coupled tocomponents near the combustion chamber.

FIG. 3 is an illustration of an aspect of the present inventionfeaturing multiple sensing grids.

FIG. 4 is an illustration of a prior art single beam gas detectionapparatus.

FIG. 5 is an illustration of a prior art multiple beam gas detectionapparatus.

FIG. 6 is an illustration of the use of an echelle grating in thepresent invention.

FIG. 7 is an illustration of an echelle grating based diode laserspectroscopy gas sensing apparatus of the present invention.

FIG. 8 is an illustration of a pitch side optical system suitable forminimizing mode noise.

FIG. 9 is an illustration of a fiber coupled gas sensing apparatus.

FIG. 10 is an illustration of light lost between pitch and catch optics.

FIG. 11 is an illustration of the angular acceptance cone of a fiberoptic system.

FIG. 12 is a schematic diagram of an alignment mechanism of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Sensing Apparatus

As shown in FIG. 1 an embodiment of the present invention is a sensingapparatus 10 suitable for the sensing, monitoring and control of acombustion process. The sensing apparatus 10 performs tunable diodelaser absorption spectroscopy (TDLAS) using laser light from a series oftunable diode lasers 12 lasing at select frequencies in thenear-infrared or mid-infrared spectrum. The output of each tunable diodelaser 12 is coupled to an individual optical fiber which may be a singlemode optical fiber 14 and routed to a multiplexer 16. As used herein,“coupled”, “optically coupled” or “in optical communication with” isdefined as a functional relationship between counterparts where lightcan pass from a first component to a second component either through ornot through intermediate components or free space. Within themultiplexer 16 the laser light of some or all of the frequenciesgenerated is multiplexed to form a multiplexed probe beam havingmultiple select frequencies. The multiplexed probe beam is coupled to apitch side optical fiber 18 and transmitted to a pitch optic 20 orcollimator operatively associated with a process chamber which, in FIG.1, is shown as a combustion chamber 22.

The pitch optic 20 is oriented to project the multiplexed probe beamthrough the combustion chamber 22. Across the combustion chamber 22 inoptical communication with the pitch optic 20 is a catch optic 24. Thecatch optic 24 is preferably substantially opposite the pitch optic 20and is operatively associated with the combustion chamber 22. The catchoptic 24 is positioned and oriented to receive the multiplexed probebeam projected through the combustion chamber 22. The catch optic 24 isoptically coupled to a catch side optical fiber 26 which transmits theportion of the multiplexed probe beam which is received by the catchoptic 24 to a demultiplexer 28. Within the demultiplexer 28 the portionof the multiplexed probe beam received by the catch optic 24 isdemultiplexed and each wavelength of demultiplexed laser light iscoupled to an output optical fiber 30. Each output optical fiber 30 inturn is optically coupled to a detector 32, which typically is aphotodetector sensitive to one of the select frequencies of laser lightgenerated and multiplexed to form the probe beam. The detectors 32generate an electrical signal based upon the nature and quantity oflight transmitted to the detector 32 at the detector frequency. Theelectrical signal from each detector 32 is typically digitized andanalyzed in data processing system 34. As discussed in detail below, thedigitized and analyzed data can be used to sense physical parameterswithin the process chamber including but not limited to theconcentrations of various gas species and the combustion temperaturewithin the combustion chamber 22. The data processing system 34 canfurther be used to send signals through a feedback loop 36 to combustioncontrol apparatus 38 and thereby actively control select processparameters. In the case of a combustion process, the process parameterscontrolled can include fuel (e.g., pulverized coal) feed rates; oxygenfeed rates and catalyst or chemical agent addition rates. The use offiber optic coupling of the electronic and optical components on boththe pitch and catch sides of the sensing apparatus 10 allows delicateand temperature sensitive apparatus such as the tunable diode lasers 12,detectors 32 and data processing system 34 to be located in a controlroom having a stable operating environment. Thus, only the relativelyrobust pitch and catch optics 20, 24 need be situated near the hostileenvironment of the combustion chamber 22.

FIG. 2 schematically depicts the overall component placement of a fibercoupled, multiplexed sensing system 40. The sensing system 40 generallyconsists of a system rack 42, a breakout box 44, a transmitter head 46having pitch optics 48, a receiver head having catch optics 52 andconnecting optical fibers. The system rack 42 is preferably located in aremote control room situated a distance, for example one kilometer, fromthe combustion chamber 54. The control room typically will have amoderate environment. The system rack 42 contains the lasers 56,detectors 58, wavelength multiplexers 60 and wavelength demultiplexers62. The system rack 42 also houses the system electronics and controlsoftware (not shown on FIG. 2). The system rack 42 may optionally housean alignment light source 64.

The optical fibers connecting the system rack 42 with the breakout box44 are typically standard single-mode telecom optical fiber. This typeof fiber is inexpensive, readily available, low-loss and allows thelaser light to be directed to various off-the-shelf telecom componentsto manipulate the light, such as optical switches, splitters, andwavelength division multiplexers. Without optical fiber coupling, thelaser light would have to be directed through free space all the way tothe combustion chamber 54, which would be very difficult to implementor, alternatively, sensitive electronic and optical components wouldhave to be situated in close proximity to the combustion chamber 54.

Also shown on FIG. 2 is a breakout box 44. The breakout box 44 is aruggedized enclosure located close to the boiler. The breakout box 44contains optical switches, splitters and couplers (collectively 66)which may be used as discussed below to direct the optical signals tomultiple transmitter-receiver head pairs.

A third group of system components as shown on FIG. 2 are thetransmitter and receiver heads 46, 50. The optics and electronics in thetransmitter and receiver heads 46, 50 must convert the light in thefiber 68 into a collimated beam, direct the beam accurately through thecombustion chamber 54, capture the beam on the far side of thecombustion chamber 54 and couple the beam into the fiber 70. The choiceof optics to accomplish this is determined by the transmission distance,the turbulence of the combustion zone, its effect on the transmittedbeam's quality, and the core size of the fiber 70. Preferably, the fibercore diameter is 50 microns, which is a compromise: a larger core willcapture more of the laser light but also much more of the backgroundlight. Fiber coupling on the catch (receiver) side has severaladvantages. In particular, only light in the same location as the laserlight and traveling in the same direction is focused into the fiber 70.This drastically reduces the amount of background light that is sensed.In an alternative embodiment, light may be captured into one of severalreceiver fibers and an optical switch or other optical routing devicecan select light from one fiber for routing to the detectors 58.

The use of fiber coupling at the catch side requires that the alignmenttolerances of both the transmitter and receiver optics be preciselymaintained (less than 0.5 milliradian for both the transmitter andreceiver pointing). The alignment system discussed below makes itfeasible to meet these tolerances in a harsh power plant environment.Preferably, both the pitch and catch optics 48, 52 are custom-designedand aberration-corrected for wavelengths from 660 nm to 1650 nm so thatmultiple laser signals can be efficiently transmitted and received atthe same time.

Sensing Apparatus With Multiple Sets of Sensing Optics

Referring again to FIG. 1, an embodiment is depicted schematically whichfeatures more than one set of pitch optics 20 and catch optics 24associated with a single combustion chamber 22. The multiplexed probebeam can be routed by a routing device which, as is shown in FIG. 1, maybe an optical switch 72 to each set of pitch optics 20. Suitable routingdevices include optical switches which may be implemented to route theprobe beam with minimal attenuation to each set of pitch/catch optics ina predetermined sequence or an optical splitter which simultaneouslyroutes a fractional portion of the multiplexed probe beam to each set ofoptics.

A similar optical routing device which, in FIG. 1, is shown as amultimode optical switch 74 can be employed on the catch side of thesystem to route the portion of the multiplexed probe beam received byeach catch optic 24 to the catch side demultiplexer 28. Although theembodiment depicted in FIG. 1 shows only two sets of pitch and catchoptics, the system can employ any number of pitch and catch opticalsets. The use of fiber coupling and a (de)multiplexed probe beam on boththe pitch and catch sides of the system allows multiple sets of pitchand catch optics to be implemented with one set of lasers 12 anddetectors 32. Without the incorporation of optical multiplexingtechniques, separate sets of lasers, detectors and fiber cables, allrequiring calibration, would be needed for each transmitter/receiverpair. As discussed in detail below, multiple transmitter/receiver pairsallow the implementation of one or more two dimensional sensing gridsover the entire combustion chamber 22 or elsewhere, such as for sensinga downstream gas process. A schematic illustration of two highlysimplified sensing grids, a fireball sensing grid 76 and a downstreamsensing grid 78 are shown in FIG. 3. In addition, the fiber-couplednature of the present invention allows readily availabletelecommunications components to be used to positive effect. Forinstance, a fiber-optics switch can be used to route the multiplexedprobe beam to different locations for measurement. 1.times.N opticalswitches with N up to 8 are readily available as off-the-shelfcomponents from a variety of suppliers. Switches with N up to 16 can becustom ordered.

A switch and multiple pairs of pitch and catch optics can be used forserial probing of a gas species at different locations throughout thecombustion chamber. For situations in which averaged results aresufficient, serial probing of different beam paths is acceptable.However, certain applications require instantaneous probing of an entiresensing grid. For example, certain combustion process flows exhibitshigh-frequency fluctuations, or the flow may only exist for a shortperiod of time, e.g. shock tubes or tunnels. In such a case a 1.times.Nsplitter may be used to divide the probe beam into N branches each ofwhich occupies a different position on the grid. Since the entire gridis illuminated simultaneously, a two dimensional analysis can begenerated very quickly. However, simultaneous two dimensional analysismay require that each component on the catch side be reproduced for eachbeam path including demultiplexers, detectors, electronics such as A/Dcards and, to some extent, computers.

Thus, embodiments featuring switches or splitters facilitate somewhatcoarse tomographic reconstruction of two-dimensional cross sections ofthe probed region. Using diode lasers to do tomography of gasconcentrations is a known technique, however significant additionalbenefit is achieved under the present invention as a result of the useof a probe beam which is wavelength multiplexed. The wavelengthmultiplexed beam allows for the simultaneous spectroscopic analysis ofmore than one absorption line. Thus TDLAS techniques which rely on morethan one line, such as temperature determination, as discussed in detailbelow, can be performed across the entire sensing grid. Both temperatureand gas species concentrations can be mapped in this way.

Specific Application of Tomography in SCR and SNCR

A specific application of coarse tomography as described above isschematically illustrated in FIG. 3 and concerns the optimization ofammonia injection in SCRs (selective catalytic reduction) and SNCRs(selective non-catalytic reduction) for the reduction of NO.sub.x fromcoal or gas fired power generation boiler effluent. In this application,a matrix of ammonia or urea injectors 80 are placed in the flow ofboiler effluent. In order to minimize the NO.sub.x concentration, anexcess of ammonia (or urea) may be added to the effluent. NO.sub.x is aheavily regulated and highly undesirable family of air pollutants. Theadded ammonia chemically reduces the NO.sub.x and forms harmlessnitrogen gas and water as products. However, the amount of excessammonia (or urea) added must be minimized because these chemicalsthemselves are toxic air pollutants and quite expensive. Typically anexcess concentration of <3-5 ppm ammonia is desired. However, thedistribution of NO.sub.x in the power plant combustion effluent is notuniform, nor is it temporally stable. In addition, one or more of theammonia injectors may foul at any given time causing a local decrease inthe ammonia concentration leading to a local bleed through in theNO.sub.x concentration. With the ability to monitor the spatialconcentration of ammonia or NO.sub.x with a downstream TDLAS grid 78sensing as described above, the present invention allows non-uniformammonia distributions to be detected and mitigated. Thus, optimizationof the ammonia injection grid 76 with two-dimensional speciesconcentrations and individual control over injectors allows theoptimization of the SCR/SNCR process. The detectors and ammoniainjectors may be linked to a data processing system providing automatedfeedback control of the ammonia injectors.

An optimized ammonia slip detection system, such as the one describedherein, should preferably include the ability to monitor NO.sub.xconcentration. NO.sub.x includes both NO and NO.sub.2. Unfortunately,robust NIR diode lasers can only access the second NO overtonetransition occurring in the 1.7-1.8-micron region. This transition istoo weak to detect NO given the relatively low concentrations present inmost effluent flows. Therefore it is not practical to directly monitorNO concentration. However, NO.sub.2 is formed by the same processes thatform NO. These processes, known in the power generation industries asthe thermal NO.sub.x process produce both NO and NO.sub.2 with NOaccounting for approximately 95% of the total NO.sub.x concentration andNO.sub.2 accounting for the remaining 5% under typical conditions. Theexact ratio typically depends upon temperature and the oxidizingpotential of the environment. As discussed above, this technique allowsfor determination of the temperature of the sampled gas as well However,it is expected that the NO and NO.sub.2 concentrations will track eachother. Thus NO.sub.2 can be used as a surrogate analysis species for NO.The present invention provides the ability to monitor NO.sub.2 at awavelength of 670 nm. This wavelength is produced using a 1340-nmdistributed-feedback (DFB) laser frequency-doubled in a phase-matchedperiodically poled lithium Niobate waveguide. Even though the NO.sub.2concentration is only 5% the NO concentration, the NO.sub.2 absorptionstrength is orders of magnitude stronger. Thus, NO.sub.2 can be detectedreadily at the concentrations present in boilers to facilitateoptimization of NO.sub.x reduction processes.

Tunable Diode Laser Absorption Spectroscopy

The present invention performs TDLAS using techniques known to thoseskilled in the art of laser spectroscopy. Generally, TDLAS is performedby the transmission of laser light through a target environment,followed by the detection of the absorption of the laser light atspecific wavelengths, due to target gases such as carbon monoxide oroxygen. Spectral analysis of the detected light allows identification ofthe type and quantity of gas along the laser path. The details of directabsorption spectroscopy are discussed in Teichert, Fernholz, and Ebert,“Simultaneous in situ Measurement of CO, H.sub.2O, and Gas Temperaturein a Full-Sized, Coal-Fired Power Plant by Near-Infrared Diode Lasers,”(Applied Optics, 42(12):2043, 20 Apr. 2003), which reference isincorporated herein in its entirety. The non-contact nature of laserabsorption spectroscopy makes it well-suited for harsh environments suchas the combustion zone of a coal-fired power plant, or flammable ortoxic environments where other probes cannot be used. The use of laserlight provides the high brightness necessary to get detectibletransmission in the presence of severe attenuation (typically greaterthan 99.9% loss of light) that may be seen in some of theseenvironments. To better withstand the harsh conditions of the targetapplications, the laser light may be brought in to the targetenvironment through armored optical fiber.

Effective sensing of temperature or multiple combustion processcomponent gasses requires the performance of TDLAS with multiple widelyspaced frequencies of laser light. The frequencies selected must matchthe absorption lines of the transitions being monitored. For example, asdiscussed above it is useful to monitor NO.sub.2 at a wavelength of 670nm to approximate emission NO concentrations. It is also quite useful tomonitor oxygen, water (temperature), and carbon monoxide in a coal-firedutility boiler. Suitable absorption lines, and thus suitable lasingfrequencies can be selected based upon an assumption that the laserprobe path length through a combustion chamber is equal to 10 meters andthat the mole fraction of each species is CO (1%), O.sub.2 (4%),CO.sub.2 (10%), and H.sub.2O (10%). For frequency selection purposes,the process temperature can be assumed to be 1800 K which is slightlyhigher than what is typically observed in a coal fired plant, but thecushion serves as a safety factor in the calculations.

For example, three water absorption lines can be selected for TDLAS thatmeet the following criteria:

1. Lower state energy of .about.1000, 2000, and 3000 cm.sup.−1respectively

2. Provides a convenient absorbance of around 0.1-0.4 that, in turn,leads to approximately 20% beam absorption on resonance.

3. The optimum situation is to utilize transitions in the 1250 to 1650nm region where inexpensive, high power, DFB diode telecommunicationslasers are available.

4. The transitions must be well separated to allow for easymultiplexing.

5. The selected wavelength must be efficiently diffracted by theexisting (de)multiplexer gratings.

Suitable water lines occur at the following wavelengths: TABLE-US-00001TABLE 1 UNP Lower Grating Wave-Wave-State Absoprtion Effi-length numberEnergy Grating at 1800K ciency (nm) (cm.sup.−1) (cm.sup.−1) Order and 10M (model) 1349.0849 7412.432 1806.67 6.87 19.7% 81% 1376.4507 7265.0623381.662 6.73 28.1% 77% 1394.5305 7170.872 1045.058 6.65 6.8% 72%

No interference from any other combustion gases is anticipated. The mostlikely species to interfere, CO.sub.2 was modeled and there are nostrong, interfering lines in the 1.3-1.4 micron region.

Similarly, a suitable carbon monoxide line can be selected based on thework of Ebert referenced and incorporated above. A suitable carbonmonoxide line is found at 1559.562 nm using the R(24) line in acoal-fired utility boiler. Selection of this line avoids interferencefrom water and carbon dioxide. Known gratings are quite efficient inthis wavelength region since it is in the optical communications C band.The absorbance at this wavelength is expected to be 0.7%.

In addition, oxygen can be measured at 760.0932 nm. The preferred(de)multiplexing grating efficiency calculates to be only 40% in thisregion, however suitable laser power should be available for reasonablemeasurement efficiency.

As discussed herein, the use of fiber coupling on both the pitch andcatch sides of a TDLAS sensing apparatus requires critical alignment ofthe pitch and catch optics. Active alignment is preferably accomplishedwith a select alignment wavelength. One possible alignment wavelength is660 nm because high power (45 mW) diodes are available at this frequencyand 660 nm would be near the peak of 14.sup.th order grating operation.Other alignment wavelengths may be determined to be equally or moresuitable.

In summary, a reasonable set of wavelengths selected for multiplexing toa probe beam for TDLAS as embodied in the present invention are as shownin Table 2. It should be noted that this wavelength set is for oneembodiment of a TDLAS sensing apparatus suitable for the sensing andcontrol of a coal fired power plant. Other wavelength sets can beequally suitable. TABLE-US-00002 TABLE 2 Purpose Wavelength (nm)Alignment 660 O.sub.2 b-a band 760.0932 H.sub.2O (moderate temp. line)1349.0849 H.sub.2O (high temp. line) 1376.4507 H.sub.2O (low temp. line)1394.5305 CO R(24) of (2, 0) overtone 1559.562

Specific Benefits of TDLAS Using Multiplexed Beam

A particular advantage of TDLAS with a wavelength-multiplexed probe beamis increased accuracy of temperature measurements. In order to makeaccurate concentration measurements with TDLAS, the temperature of themonitored gas must be known. The strength of a molecular absorption is afunction of temperature. Thus, to convert the amplitude of an absorptionfeature to concentration, the temperature must be known. Certainprevious attempts to measure the concentration of combustion speciessuch as CO suffer from insufficiently accurate temperature measurementsleading to errors in quantification. This is particularly true for diodelaser based ammonia slip monitors that have traditionally notincorporated temperature measurement at all. In the sensing system ofthe present invention, temperature may be determined by measuring theratio of the intensity of two or more molecular water lines. The ratioof the integrated intensity of two lines is a function of temperatureonly (assuming constant total system pressure). Thus, in principle, twolines provide an accurate temperature. However, in the case of anon-uniform temperature distribution (as is typically found within anindustrial combustion process), two lines do not suffice to determinethe temperature distribution. In such a non-uniform temperaturedistribution, two lines can only determine a “path-averaged”temperature. In contrast, measuring the integrated amplitude of morethan two lines (of the same species) allows temperature non-uniformityto be probed. An example of this technique has been demonstrated usingoxygen as the probe molecule by Sanders, Wang, Jeffries and Hanson in“Applied Optics” (vol. 40, num. 24, 20 Aug. 2001), which reference isincorporated herein in its entirety. The preferred technique relies onthe fact that the distribution of peak intensities measured along a lineof sight is not the same for a path at an average temperature of 500 K,for example, as it is where one half of the path is at 300 K and theother half is at 700 K.

In addition to the benefit of more accurate temperature measurement, theuse of a multiplexed probe beam can allow for the simultaneousmonitoring of more than one combustion gas species, allowing for morerefined control over the combustion process.

Echelle Grating Based Apparatus

The present invention benefits from the use of relatively inexpensiveand commonly available optical components designed for use in thetelecommunications industry. The telecommunications apparatus serve wellto fiber couple the pitch and catch sides of the system.Telecommunications applications typically use optical multiplexers whichaccept multiple light beams at wavelengths which are relatively closespaced and separated by a constant value (such as 0.8 nm). The lightbeams are then generally coupled onto a single-mode optical fiber.Demultiplexers perform the inverse process. Telecommunications equipmentis typically designed to perform at wavelengths between 1520 and 1620nm, with the Optical C-band, 1528-1563 nm, being the most utilized.

In the case of (de)multiplexers, the same physical device may be usedfor either multiplexing or demultiplexing, depending upon the directionof the light which passes through it. Consequently, the term“multiplexer” or “mux” as used herein will be understood to include bothmultiplexing and demultiplexing functions.

Optical multiplexers may use any of several technologies to accomplishthe mux/demux function. However, echelle grating-based muxes areadvantageous in that they may be incorporated into a very simple andcompact design. Echelle gratings are relatively course diffractiongratings that operate in orders other than the first order with blazeangles typically greater than 45 degrees. The course line spacing on thegrating combined with high order operation results in a large angulardispersion that allows the device to be compact.

Some telecommunications applications may require that other wavelengthswell outside of the C band be simultaneously serviced by opticalmultiplexers (for instance 1310 nm). Additionally, applications outsidethe telecommunications field such as the TDLAS sensing and controlapparatus of the present invention may require the multiplexing of laserlight at widely separated wavelengths, such as separations ofapproximately 100 s of nm. An example illustrative of the benefits of anechelle grating based multiplexed sensing apparatus is developed inFIGS. 4-7. FIG. 4 illustrates a gas sensing apparatus 82 in which light84 is directed through a flame 86 from one side. A sensor 88 on theother side of the flame detects the transmitted light and determines howmuch light is absorbed by the gasses in the flame 86. In the deviceillustrated in FIG. 4, only a single beam of light is passed through theflame 86. The wavelength of the light may be chosen to correspond to theabsorption wavelength of a particular gas. Alternatively, the light maybe a white light which, after passing through the flame 86, is spit intovarious wavelengths, such as by a prism. The absorption at eachwavelength of interest can then be measured.

In a slightly more sophisticated prior art alternative, a device such asthat illustrated in FIG. 5 may be used to pass a number n of separatebeams of light 90A-90n through the flame 92. Each beam of light 90A-90nis at a different wavelength of interest and sensors 94A-94n on theother side of the flame 92 measure the absorption at each wavelength,indicating relative amounts of selected gases of interest. There areseveral drawbacks to using multiple separate beams. First, access to theflame may be limited and attempting to project the multiple beamsthrough the limited space may be awkward, if not impossible. Second,there is typically turbulence in the flame as well as pockets ofnon-uniformity. The multiple beams, even if very closely spaced, may notpass through the same sampling space and therefore not generateconsistent or comparable results. Finally, the projection and sensingoptics and detectors are more complex and costly in a multiple-beamapparatus compared to those in a single-beam apparatus.

The echelle grating multiplexer based sensing device of the presentinvention has significant advantages over the prior art. An echellegrating provides unusual flexibility by being able to operate in ordersother than the first order with blaze angles typically greater than 45degrees. For instance, the Zolo Technologies, Inc. Zmux.™. is optimizedto operate in a Littrow configuration in the 6.sup.th order at 1545 nmwith a mechanically ruled grating having a line spacing of 171.4lines/mm and a blaze angle of 52.75 degrees. The grating equation for aLittrow mount is: m.lamda.=2d sin .theta.sub.b (1) where m is the order,.lamda is the wavelength, d is the spacing between rulings, and.theta.sub.b is the blaze angle.

For a given grating, m.lamda. is a constant. For the Zmux gratingreferenced above, m.lamda.=6(1.545)=9.27 microns. Such a gratingprovides optimum efficiency for 1.545 microns in 6.sup.th order.However, the grating also provides very high efficiency for other ordersas well. For instance, the 7.sup.th order occurs at 9.27/7=1.32 microns.Thus, a grating such as the Zmux can simultaneously multiplex C bandlight as well as 1310 nm light with high efficiency. FIG. 6 illustratesan echelle grating 96 multiplexing light 98A-98n to be collimated by acollimator 100 into a single beam 102.

Of particular relevance to the present invention are applicationsoutside of the telecommunications field for multiplexing laser light atwidely separated wavelengths, often over 100 s of nm. In applicationssuch as the present TDLAS based gas-sensing apparatus, it is criticalfor all wavelength components of a probe beam to sample the same regionof space and many wavelengths may be necessary to detect a singlespecies or to detect multiple species. For these applications, echellegrating-based mux/demuxes provide a unique solution. For example, theechelle based de/multiplexer described above is capable of muxing asubstantial wavelength region about the central wavelengths given inTable 3 below where each wavelength region corresponds to a differentgrating order. TABLE-US-00003 TABLE 3 Approximate Central WavelengthWavelength Order (microns) Range (microns) 2 4.63 4.40-4.80 3 3.092.90-3.30 4 2.32 2.15-2.40 5 1.85 1.70-1.95 6 1.55 1.50-1.57 7 1.321.24-1.39

Higher orders up to and beyond the fourteenth order may also bemultiplexed with correspondingly narrower wavelength ranges. It isnoteworthy that single mode transmission for all of these wavelengths isnot possible in readily available optical fiber. One aspect of agas-sensing apparatus 104 of the present invention is schematicallyillustrated in FIG. 7. This aspect highlights the advantages of TDLASwith a multiplexed laser output has over the prior art embodimentsdiscussed above. In the FIG. 7 embodiment, a number n of laser sources106A-106n operating at widely separated wavelengths are multiplexed byan echelle grating 108 onto a single optical fiber 110. The light fromthe single fiber 110 is collimated by a collimator or pitch optic 112and passed through the sample 114 (such as a flame) to be analyzed.After passing through the sample 114, the light is demultiplexed byanother echelle grating 116. The transmitted light at each wavelength isdetected by a corresponding photodetector 118A-118n. The lasers106A-106n are tuned over a narrow spectral region (such as 1-2 nm) andabsorption by the sample 114 is monitored over each spectral regionscanned. In this way, the gas under test can be fully identified andquantified. Other parameters such as gas temperature and pressure may bemeasured as well. In addition to combustion sensing, widely separatedwavelength echelle grating mux/demux technology may enable medicaldevices to measure gases in exhaled breath and homeland defense devicesto detect chemical warfare agents. Other applications are possible inthe fields of display and microscopic vision technology by using theechelle based muxes and red/green/blue coupler devices.

Mode Noise

The optical train of the TDLAS system of the present invention, andsimilar implementations which require a signal multiplexed from widelyspaced wavelengths presents many design challenges due to the opposingdesign requirements of the reduction of mode noise and high efficiencylight collection. Mode noise is defined herein as a change in the signallevel of detected light that results from non-uniform time andwavelength varying light distribution in the core of a fiber used tocollect and transport the light to and from the process chamber beingmeasured.

In a multimode fiber, different modes propagate at different velocitiesdue to refractive index variations. The intensity distribution in thefiber is then a speckle pattern resulting from interference of all thepropagating modes that have undergone different effective path lengths.If all light in the speckle pattern is collected and detected, thenconstructive and destructive interference cancel exactly and the totaltransmitted power does not depend on wavelength or fiber length. Ifclipping, vignetting or other loss is introduced, the exact cancellationfails and the detected power changes with wavelength and/or time. Ageneral expression for the detected power after a length, z, of fiberis:

-   .rho.=P.sub.O+.SIGMA.sub.ijc.sub.ijE.sub.iE.sub.j cos-   [(2.pi.nu.sub.0.DELTA.n.sub.ijz)/c+.DELTA.phi.sub.ij    (T,.sigma.)] (2) where

P.sub.0=wavelength independent average power

E.sub.i=amplitude of light in the ith transverse mode

c.sub.ij=overlap integral between the ith and jth transverse mode

.DELTA.n.sub.ij=refractive index difference between ith and jth modes

DELTA.phi.sub.ij=phase shift between ith and jth modes due totemperature and stress

For an orthonormal set of modes and no loss, c.sub.ij=0. However, withany beam clipping or vignetting or any other mode dependent loss willcause some c.sub.ij.noteq.0. This will lead to ripples in the averagetransmitted power.

For a typical graded-index fiber with a 50 micron core, the total indexchange, .DELTA.n, is .about.1%, but most modes spend the bulk oftransmission time close to the fiber core center, and therefore,.DELTA.n.sub.ij.1toreq.0.0005, in general. The commonly availableoptical fiber GIF50 supports approximately 135 modes, which issufficiently coarse to produce prominent mode noise during a wavelengthscan given reasonably achievable beam clipping levels.

As a concrete modal noise example, one may consider the simplestpossible system that exhibits mode noise: a rectangular waveguidesupporting only the lowest mode in one dimension and only the two lowestmodes in the orthogonal dimension: Lowest mode:

-   E.sub.1=E.sub.1.sup.0[exp i(kz−.omega.t)]cos.pi.x/2a Next mode:    E.sub.2=E.sub.2.sup.0[exp i(kz−.omega.t)]sin.pi.x/a The intensity at    a point z along the fiber is:    I(x)=|E.sub.1+E.sub.2|.sup.2 and the total power is    P=.intg.|E.sub.1+E.sub.2|.sup.2dx (3) where the integral must    include the effects of clipping and vignetting.

In the absence of clipping, P.about.E.sub.1.sup.2+E.sub.2.sup.2 andthere is no wavelength dependence. Adding clipping amounts to changingthe limits of the integral. It can be shown that clipping results in anadditional term .about.E.sub.1E.sub.2cos DELTA.phi. where.DELTA.phi.=.DELTA.kL=2.pi.DELTA.nL/.lamda.

If single-mode fiber could be used in the catch side optical train ofthe present invention, mode noise would not be an issue. However,multimode fiber must typically be used in the catch side optical trainof the present invention for two reasons. First, after traversing themeasurement volume (a combustion chamber with a measurement path inexcess of 10 meters), the initially single-mode (Gaussian spatialdistribution) beam is significantly degraded in quality. Thus, thecoupling efficiency of this severely distorted beam into single-modefiber would be very poor. This is an unacceptable situation since thebeam is attenuated by 3-4 orders of magnitude when passing through themeasurement volume primarily due to scattering and obscuration by sootand fly ash. The additional attenuation resulting from using single-modefiber would preclude measurement. Second, refractive beam steeringeffects in the fireball cause the position and pointing of the beam tobe unstable. Given these effects, it would be difficult to “hit” thecore of a single-mode fiber with any regularity.

On the other hand, the core of a multimode fiber presents at least 25times the target cross-sectional area of a single-mode fiber. Thus, theeffects of beam steering can be significantly reduced. In addition,since the coupling efficiency into multimode fiber is independent of thespatial mode of the light, the poor beam quality obtained after passingthrough the fireball is not an issue.

However, mode dependent losses occurring in the multi-mode fiber trainare a significant design challenge. The light distribution emanatingfrom the core of a multimode fiber exhibits a random speckle pattern,i.e. a random pattern of light and dark areas caused by constructive anddestructive interference between different modes of the fiber. If thespeckle pattern was totally invariant as a function of time andwavelength, it would not present a problem. However, slow variations inthe speckle pattern particularly as a function of wavelength can causemode noise if the beam is clipped anywhere in the multimode catch sideoptical train as described above. This clipping is impossible to avoid;it can only be reduced. Therefore, additional measures to reduce modenoise must be implemented to improve the detection sensitivity of thesystem.

There are several ways in which to mitigate mode noise. From equation 2above, mode noise may be reduced by:

1) reduce mode dependent losses, i.e. reduce clipping thereby keepingthe c.sub.ij small

2) reduce z, thereby increasing the period of the model noise to be muchgreater than the absorption lines of interest

3) reduce .DELTA.n.sub.ij by using low dispersion fiber

4) scramble the modes; but not all mode scrambling techniques areequally effective, as is described below.

Preferably, the catch optics of the present invention are designed andimplemented to incorporate all of the above in order to reduce modalnoise. The optics are designed such that any beam clipping should occurat a low level given near perfect alignment of the system. Effortsshould be made to keep the length of multimode fiber to a minimum;however, for some applications z must be long in order to have thecontrol electronics in an environmentally controlled area. The value of.DELTA.n.sub.ij may be reduced by using premium low-dispersion multimodefiber. In addition, excellent results may be obtained by scrambling themodes by mechanical manipulation of a catch side multimode fiber.

The speckle pattern exhibited in a multimode fiber varies as a functionof time and wavelength and also as a function of the mechanical positionof the fiber. Flexing the fiber and manipulating it in specific ways cancause the speckle pattern to change. If these mechanical manipulationsare performed continuously, over a period of time, the spatialdistribution of light emanating from the fiber averages to a relativelyuniform pattern. The crux of the scrambler of the present invention isto reduce mode noise by mechanically manipulating multi-mode fiber toproduce, on average, a uniform light beam that does not produce modenoise when subjected to inevitable low-level beam clipping.

Some specific modes of fiber manipulation are more effective at reducingmode noise than others. In particular, twisting the fiber about itslongitudinal (z) axis relative to some other point on the fiber causesthe speckle pattern to change. In particular, the dominant changeobtained is a rotation of the speckle pattern around the z-axis. Ofinterest is the fact that the pattern does not rotate as far around theaxis as the fiber is mechanically rotated. A secondary effect is thatthe actual light distribution is somewhat altered by the rotation. Therotation of the speckle pattern is not due to stress-induced refractiveindex changes in the fiber, although this may explain small changes inthe speckle intensity pattern. Rather, the rotation is due to thelight's inability to completely follow the waveguide as it ismanipulated in a torsional motion.

This observation can be used to virtually eliminate mode noise caused bythe use of multimode fiber for the catch side optical train. A highlypreferred embodiment of the present invention uses a hollow shaft motorthrough which the multi-mode fiber is placed and fastened. A remotesection of fiber is held fast relative to the shaft position of themotor, and the motor is repetitively swept through +360 degrees and then−360 degrees of motion. The frequency of this motion preferably isgreater than or equal to 10 Hertz to enable effective averaging of thetransmitted signal, and significantly reduce the effect of catch sidemode noise.

The pitch side optical train of the present invention also presents asignificant design challenge due to the necessity of producing asingle-mode beam for all wavelengths to be transmitted through themeasurement region. If single-mode fiber could be used throughout thepitch side optical train, mode noise would not be an issue. However,fiber only operates as a single-mode waveguide over a limited wavelengthwindow. Beyond the short wavelength cutoff for a particular fiber, lightcan be transported through the fiber in several higher order spatialmodes. These higher order modes will interfere to produce a specklepattern when the light exits the fiber. The speckle pattern is time andwavelength-varying. Even a small amount of beam clipping then gives riseto noise in the measurement.

On the contrary, if a fiber is selected that has a single-mode cutoffthat matches the shortest wavelength that needs to be transmitted, thelonger wavelengths will suffer a substantial loss when coupled into thefiber and the fiber will exhibit extensive bending losses for the longerwavelengths.

This problem can be acute in the fiber coupled, wavelength multiplexedTDLAS sensing and control device of the present invention due to theneed to multiplex wavelengths as long as 1.67 microns with wavelengthsas short as 760 nm or 670 nm. There is no known single commerciallyavailable fiber that will provide single mode operation, high couplingefficiency and low bending losses for such a broad range of wavelengths.Photonic crystal fiber may in the future provide a solution to thisdilemma, but Photonic crystal fiber technology is currently in itsinfancy.

As shown in FIG. 8, the current invention alleviates the problem ofmultiplexing and pitching light in a single mode beam from 670 m or 760nm to 1.67 microns by utilizing a very short transmission section ofmultimode fiber 120 that does not allow the higher order spatial modesfor a wavelength shorter than the single mode cutoff to develop.Referring to equation 2 above, if the length, L, of multi-mode fiber isshort, then mode noise will be minimized. In this case, for example, if760 nm light is coupled to a short section of single mode fiber with acutoff wavelength of 1280 nm (e.g. Corning SMF 28), the 760 nm lightremains single-mode for at least a few meters. Therefore a solution topitch side mode noise is to couple the 760 nm light into a fiber whichis single mode for wavelengths longer than 1280 nm but could bemultimode for 760 nm, with only a short distance to go before it iscollimated to be transmitted through the measurement zone.

A schematic diagram of such a system is shown in FIG. 8 and FIG. 2.Referring first to FIG. 8, multiple diode laser sources 120 lasing atwidely spaced lasing frequencies are coupled to discrete single modeoptical fibers 122A-122n. The diode lasers lasing at wavelengths between1349 nm and 1670 nm are multiplexed with multiplexer 124. The output ofmultiplexer 124 is coupled to a pitch side fiber optic 126 havingsuitable dimensions for transmitting light with wavelengths ranging from1349 nm-1670 nm, both without substantial transmission losses andwithout the introduction of mode noise. A suitable fiber optic for thesewavelengths is Corning SMF28. However, the 760 nm input, if multiplexedand coupled to an SMF28 optical fiber would, after transmission over arelatively short distance, become multimodal. Accordingly, the output ofthe 760 nm laser is coupled to a fiber which is single mode forwavelengths less than 1280 nm such as SMF750. The laser lighttransmitted in the input fiber 122n and the multiplexed laser lighttransmitted in the pitch side optical fiber 126 can be coupled nearbythe pitch optic 128. The coupler 130 and pitch optic 128 are preferablyoptically connected by a short length of transmission optical fiber 132with the transmission optical fiber 132 being selected to transmit allof the coupled and multiplexed wavelengths without significant loss. Asuitable transmission optical fiber for the system depicted in FIG. 8would be Corning SMF28. Provided that the transmission optical fiber isrelatively short, the 760 nm laser light coupled to the transmissionoptical fiber 132 will not exhibit multimodal behavior. For the systemand fibers depicted in FIG. 8, it has been determined that thetransmission optical fiber must be kept to a length of 3 meters or lessto avoid the introduction of significant multimodal noise.

A similar system is shown in FIG. 2 where coupler 136 receives inputfrom both a 760 nm diode laser and a multiplexed beam from diode lasershaving substantially longer wavelengths.

Alignment System

Preferably, the sensing system of the present invention incorporates anauto alignment feature that allows the pitch and catch optics tomaintain optimal alignment even though they are bolted on to a boiler orother hostile process chamber which is, itself, subject to movement fromthermal effects, wind and vibration. In a highly preferred embodiment ofthe present system, both the pitch and catch optics are mounted onfeedback-controlled tip/tilt stages. The requirement that both pitch andcatch optics be mounted on tip/tilt stages results from the fact thatthe sensor is totally fiber-coupled. Thus, multiplexed light is launchedacross the measurement region by a collimating pitch optic attacheddirectly to an input fiber, and the catch optic couples the transmittedlight directly into an output fiber that typically is a multimode fiber.Accordingly, the catch optic must be oriented so that it is collinearwith the beam emanating from the pitch optics. This is necessary so thatthe focused transmitted beam will arrive within the acceptance cone ofthe multimode catch fiber.

In order to discriminate transmitted laser light from intense backgroundlight (for example, from the flames in a coal furnace), the field ofview and focus of the detector can be limited to sense only light withthe same direction and position as the input laser light. This may bedone conveniently by focusing detected light into an optical fibercoupled to a suitable detector. The basic optical system design of anembodiment of the present invention is shown schematically in FIG. 9.The transmitter 136 of FIG. 7 consists of a pitch optic 138 orcollimator such as a collimating lens of one or more than one layer andassociated mounting and alignment structures and electronics. Similarly,the receiver 140 of FIG. 7 consists of a catch optic 142 or collimatorof similar or varied construction from the pitch optic and associatedmounting and alignment electronics. The efficiency and backgrounddiscrimination of a transmitter-receiver pair are tied to alignmenttolerance. For highest efficiency and discrimination, the alignmenttolerances for both the transmitter and receiver are severe. Thetransmitter must be pointed accurately enough so that most of thetransmitted light strikes the clear aperture of the catch optic 146, asindicated in FIG. 10. For a typical system, this amounts to a 1 cmtolerance over a typical transmission distance of 10 meters, or 1milliradian. (With target distances between 5 and 30 meters and launchedspot sizes between 1 and 3 cm, diffraction is a small contribution.)

As is graphically illustrated in FIG. 11, the receiver's angularacceptance is determined by the fiber core 148 diameter divided by thecatch optic 150 focal length. A shorter focal length will increase theangular acceptance, but the receiver clear aperture becomescorrespondingly smaller. A compromise having adequate clear aperture andangular acceptance is to use a 50 mm focal length lens and a 50micrometer core fiber. This results in a 2 cm clear aperture and a1-milliradian cone of angular acceptance.

A preferred alignment system must therefore position two optics to pointat each other with tolerances of 1 milliradian in both tip and tilt, fora total of four degrees of freedom. These four degrees of freedom mightpossibly be accomplished by rough alignment of one side followed by afour-dimensional alignment (tip, tilt, and lateral position in x and y)of the other side, but this assumes that large lateral motions arepermissible. Since access ports for target environments may be as smallas 1 inch, this is potentially a problematic solution. Alternatively,active pointing alignment at both ends can ensure proper alignment whenlimited space is available for lateral movement.

The critical alignment of the pitch and catch optics must be maintainedin a harsh and variable environment. Vibration, wind loading,temperature change and other structural shifts may all lead tomisalignments, as will mechanical creep in the transmitter and receiveroptomechanics. Misalignment can also be expected after periodicmaintenance, when the transmitter and receiver heads are removed forcleaning and then re-mounted. Ideally, the optical system of the presentinvention will be able to maintain its 1 milliradian optical alignmentin the face of system misalignments that could approach 50 milliradians.The alignment system should also retain position during power outagesand tolerate total loss of signal, and be turned off without losingalignment. Finally, the system itself must preferably be rugged enoughto function continuously for an extended period of time in an exposed,industrial environment.

FIG. 12 schematically illustrates an embodiment of alignable pitch orcatch optics. The transmitter and receiver are similar in design: thetransmitter generates a collimated beam of laser light from lightemerging from an optical fiber, and the receiver captures a collimatedbeam of laser light and focuses it into a fiber. (It is possible to sendlight backward through this optical system, and most of the elements ofthe transmitter and receiver are identical.) The transmitter andreceiver optics may be mounted in NEMA-4 enclosures to protect them fromthe environment. The following description applies to either thetransmitter or receiver module.

As shown in FIG. 10, the pre-collimated fiber/lens pair 152 is attachedto a kinematic tilt stage 154 positioned to allow tip and tiltperpendicular to the optical axis. Two direct-drive stepper motors 156accomplish tip and tilt. These motors are controlled by computer via anEthernet or similar connection. This connection may be through opticalfiber in order to avoid electrical interference. The stepper motors 156hold their position when power is removed, so the optical alignment isunaffected by electrical power outages.

During periodic or continuous system alignment, the control computermonitors the amount of laser light that is transmitted and detected.Preferably, a discrete alignment wavelength may be provided forcontinuous or periodic alignment proceedings such as the visible lightsource 64 of FIG. 3. Any misalignment will reduce this detected signal.In auto-alignment mode, the computer measures the detected signal,directs one of the two stepper motors to move a small amount in onedirection, then re-measures the detected signal. If the signalincreases, then the computer directs the stepper motor to move again inthe same direction until the signal does not increase. The computer thendirects the other stepper motor to move along the orthogonal axis tomaximize the detected signal, then repeats the whole process for theother sensor head. As the detected signal increases, the detectoramplifier gain automatically decreases so that the auto-alignmentprocess proceeds over several decades of signal size. The auto-alignmentsystem can function with detected powers from nanowatts to milliwatts.

This “hill-climbing” algorithm is able to align the system afternear-total loss of signal, in the presence of substantial noise, and istolerant of beam blockages, power outages, mechanical shocks and otherdisturbances that could cause other alignment systems to misalign to thelimits of the control electronics. All that is required forauto-alignment is a finite signal with a global maximum in positionspace. Depending on specific installation conditions, auto-alignment mayoccur periodically at set intervals such as every hour or as neededafter an extended period such as days of operation. The control computermay monitor the detected signal and auto-align only when the signaldrops below a pre-set threshold.

The transmitter and receiver modules may incorporate several otherfeatures useful for industrial applications. Optional sensors may detectwhen the modules are moved out of position for cleaning or maintenance,and all lasers are turned off for safety. As shown on FIG. 10, allelectrical and optical connections may made through a hinge 158 so thatall such connections are undisturbed by maintenance operations. Theenclosure 160 protects the sensitive internal optics from contaminants,and is preferable hosable. The hinge range of motion may be restrictedto avoid operator injury. Preferably, each sensor head will require lessthan 10 watts of input electrical power during auto-alignment, and lessthan 0.1 watt after auto-alignment is complete.

Alternate designs may be suitable for different applications.Considerable size and weight reduction are possible if NEMA-4 enclosurerating is not required. Different transmission distances may allowoptics of different focal length and clear aperture to optimize thecaptured signal. As an alternative to the stepper motor-driven tiltstage described above to control pointing by moving the entirefiber-lens assembly, the fiber may be moved laterally relative to thelens to effect the same pointing change while moving a much smallermass. Different electromechanics such as piezoelectric elements or voicecoils may also be used to increase the speed of the auto-alignmentsystem.

In addition to the alignment concerns discussed above, the specificchoice of pitch and catch optics can affect the performance of the TDLASsensing system of the present invention in several ways:

(1) Signal strength coupled to the detectors depends on pitch/catchefficiency.

(2) The amount of undesirable background emission coupled to thedetectors depends on catch optics etendue.

(3) The effects of few-mode noise at 760 nm are very sensitive to thepitch/catch configuration.

(4) The noise characteristics (small but more steady or larger with wildswings) are expected to depend on the launched beam size. A largerlaunch beam is preferable.

(5) The system misalignment sensitivity is a direct function of pitchand catch focal lengths and associated fiber sizes.

A very simplistic picture of a typical coal fired power plant combustionzone can be adopted for optical component selection analysis. Thepurpose of such analysis is to focus on the general effects on a laserbeam passing through the fireball with as little knowledge of thefireball details as possible. Traversing the fireball has three effectson a light beam:

(1) Soot particles absorb some of the light.

(2) Large-angle refraction or scattering prevents some of the light fromreaching the catch optic.

(3) Light passes through numerous small thermal gradients and is thussteered randomly but still reaches the catch optic.

Only the third category of light is available for collection. Assumingthat a typical ray of light undergoes multiple refracting events whiletraversing the fireball, the direction of the ray follows a random path,and can drift away from its initial direction. If the ray is part of alarger beam composed of other rays that undergo similar but notidentical drifts, then the effect of the fireball on the beam causesfour distinct types of changes:

(1) Change in the overall direction of the entire beam

(2) Change in the position of the centroid of the beam

(3) Change in the beam size

(4) Change in beam divergence/wavefront flatness

These four types of changes are parameters which include all the majoreffects on the beam that will affect collection efficiency, withoutregard for the physics leading to these effects.

If the directions of the rays of light in the fireball are undergoingrandom drift then the ray directions may diffuse away from the initial(nominally optimum) direction according to a standard diffusiondependence. However, the distance of a ray of light from the originalaxis depends on its prior directions. Thus, for a given amount of rmsbeam steering, determined by the details of the fireball, a directlyproportional amount of beam offset can be expected. If a laser beamtraversing the fireball is blown up to several times its original sizethen the same relationship will hold between the final beam size andfinal beam divergence.

Light collection efficiency can be estimated if we know the angle,location and beam size of the light incident on the catch optics. Theestimate assumes simple ray optics, flat-top intensities and a simpleestimate the amount of light incident on the catch optic that is withinthe fiber numerical aperture NA and which is boresighted to strike thecore of the fiber. The end result is a “hill” in offset-angle space.Assuming optimum alignment, collection efficiency is close to the top ofthe hill but may be rapidly moving over and around the hill as beamangle and position fluctuate. Preferably, the collection efficiency hillwill be both as tall and as broad as possible. Several points about thehill are worth noting:

(1) The peak height of the collection efficiency hill (for zero beamoffset and tilt) is proportional to the square of the catch opticetendue (focal length times NA) unless the catch optic is large enoughto capture the entire incident beam, at which point the catch efficiencyis 100%.

(2) The hill is elliptical, and changing the catch focal length makesone axis longer and the other shorter.

(3) The fluctuation in light collection efficiency due to beam jitter isa noise source.

Based upon the foregoing analysis techniques, the signal to noise ratioof different pitch/catch combinations may be compared. Assuming that themultiplicative noise is the same, different catch optics differ in finalperformance only if the fireball background noise or detector noise aredominant.

The objects of the invention have been fully realized through theembodiments disclosed herein. Those skilled in the art will appreciatethat the various aspects of the invention may be achieved throughdifferent embodiments without departing from the essential function ofthe invention. The particular embodiments are illustrative and not meantto limit the scope of the invention as set forth in the followingclaims.

1. A catch side optical system for use in diode laser spectroscopy comprising: a catch optic optically coupled to a proximal end of a catch side multimode optical fiber; and means to mechanically manipulate a section of the catch side multimode optical fiber to reduce catch side mode noise.
 2. The catch side optical system of claim 1 wherein the catch side multimode optical fiber has a longitudinal axis parallel to the fiber length and the mechanical manipulation comprises twisting the catch side multimode optical fiber around the longitudinal axis.
 3. The catch side optical system of claim 2 wherein the means to mechanically manipulate the section of the catch side multimode optical fiber comprises a motor operatively associated with the catch side multimode optical fiber such that the section of fiber is attached relative to a shaft position of the motor, and the motor shaft is repetitively swept through +360 degrees and −360 degrees of motion.
 4. The catch side optical system of claim 3 wherein the frequency of the motor shaft sweep enables effective averaging of the transmitted signal, and thereby reduce the effect of catch side mode noise.
 5. The catch side optical system of claim 4 wherein the frequency of the sweep shaft is equal to or greater than 10 Hertz.
 6. A method of coupling light in a diode laser spectroscopy system comprising: receiving light in a catch optic optically coupled to a proximal end of a catch side multimode optical fiber; and mechanically manipulating a section of the catch side multimode optical fiber to reduce catch side mode noise.
 7. The method of coupling light in a diode laser spectroscopy system of claim 6 wherein the catch side multimode optical fiber has a longitudinal axis parallel to the fiber length and the mechanical manipulation comprises twisting the catch side multimode optical fiber around the longitudinal axis.
 8. The method of coupling light in a diode laser spectroscopy system of claim 7 wherein the means to mechanically manipulate the section of the catch side multimode optical fiber comprises a motor operatively associated with the catch side multimode optical fiber such that the section of fiber is attached relative to a shaft position of the motor, and the motor shaft is repetitively swept through +360 degrees and −360 degrees of motion.
 9. The method of coupling light in a diode laser spectroscopy system of claim 8 wherein the frequency of the motor shaft sweep enables effective averaging of the transmitted signal, and thereby reduce the effect of catch side mode noise.
 10. The method of coupling light in a diode laser spectroscopy system of claim 9 wherein the frequency of the sweep shaft is equal to or greater than 10 Hertz. 